Download (i) Transformer sub-stations.

Sub Station
Introduction
• The electric power is produced at the power
stations which are located at favorable places
delivered to the consumers through a large
network of transmission and distribution.
• it may be desirable and necessary to change
some characteristic (e.g. voltage, a.c. to d.c.,
frequency, p.f. etc.) at many places in the line.
• This is accomplished by suitable apparatus
called substation.
Sub-Station
• The assembly of apparatus used to change some
characteristic (e.g. voltage, a.c. to d.c., frequency,
p.f. etc.) of electric supply is called a sub-station.
• It should be located at a proper site. As far as
possible, it should be located at the centre of
gravity of load.
• It should provide safe and reliable arrangement,
facilities for carrying out repairs and maintenance
• It should be easily operated and maintained
• It should involve minimum capital cost.
Classification of Sub-Stations
According to service requirement
 to change voltage level or improve power
factor or convert a.c. power into d.c. power
etc
(i) Transformer sub-stations.
• change the voltage level of electric supply
• Most of the sub-stations in the power system
are of this type
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(ii) Switching sub-stations
do not change the voltage level i.e. incoming
and outgoing lines have the same voltage.
However, they simply perform the switching
operations of power lines.
(iii) Power factor correction sub-stations
improve the power factor of the system
generally located at the receiving end of
transmission lines.
generally use synchronous condensers as the
power factor improvement equipment.
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(iv) Frequency changer sub-stations
change the supply frequency
frequency change may be required for
industrial utilization.
(v) Converting sub-stations
change a.c. power into d.c. power to supply
for such purposes as traction, electroplating,
electric welding etc.
(vi) Industrial sub-stations
supply power to individual industrial concerns
 According to constructional features.
(i) Indoor sub-stations.
• For voltages upto 11 kV, the equipment of the sub-station is
installed indoor because of economic considerations.
• When the atmosphere is contaminated with impurities,
these sub-stations can be erected for voltages upto 66 kV.
(ii) Outdoor sub-stations
• For voltages beyond 66 kV, equipment is invariably installed
outdoor.
• The clearances between conductors and the space required
for switches, circuit breakers and other equipment
becomes so great that it is not economical to install the
equipment indoor.
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(iii) Underground sub-stations
When the space available for equipment and
building is limited and the cost of land is high.
Under such situations, the sub-station is created
underground.
(iv) Pole-mounted sub-stations
This is an outdoor sub-station with equipment
installed overhead on H-pole or 4-pole structure.
It is the cheapest form of sub-station for voltages
not exceeding 11kV (or 33 kV in some cases)
Comparison between Outdoor and
Indoor Sub-Stations
Transformer Sub-Stations
• Transformer sub-stations may be classified
into :
 (i) Step-up sub-station
 (ii) Primary grid sub-station
 (iii) Secondary sub-station
 (iv) Distribution sub-station
Pole mounted substation
Symbols for Equipment in Sub-Stations
Equipment in a Transformer SubStation
1. Bus-bars
• When a number of lines operating at the same
voltage have to be directly connected electrically,
bus-bars are used as the common electrical
component.
• The incoming and outgoing lines in a sub-station
are connected to the bus-bars.
• The most commonly used bus-bar arrangements
in sub-stations are :
(i) Single bus-bar arrangement
(ii) Single bus-bar system with sectionalisation
(iii) Double bus-bar arrangement
2. Insulators
• They support the conductors (or bus-bars) and
confine the current to the conductors
• Material for the insulators is porcelain
3. Isolating switches
• It is often desired to disconnect a part of the system
for general maintenance and repairs. This is
accomplished by an isolating switch or isolator
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4. Circuit breaker
A circuit breaker is an equipment which
can open or close a circuit under normal
as well as fault conditions.
It is so designed that it can be operated
manually (or by remote control) under
normal conditions and automatically
under fault conditions.
A relay circuit is used with a circuit
breaker.
Bulk oil circuit breakers are used for
voltages upto 66kV while for high (>66
kV) voltages, low oil circuit breakers are
used.
Air-blast, Vacuum or SF6 circuit breakers
are used for Higher Voltages
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5. Power Transformers
A power transformer is used in a
sub-station to step-up or stepdown the voltage.
Either to use 3-phase
transformers in sub-stations or 3
single phase bank of
transformers can also be used
Advantages of 3-phase
transformer instead of 3 single
phase bank of transformers
only one 3-phase load-tap
changing mechanism can be
used.
its installation is much simpler
than the three single phase
transformers.
6. Instrument transformers
• The function of these instrument
transformers is to transfer voltages
or currents in the power lines to
values which are convenient for the
operation of measuring instruments
and relays.
(i) Current transformer (C.T.).
• It is a step-up transformer which
steps down the current to a known
ratio.
• The primary of this transformer
consists of one or more turns of
thick wire connected in series with
the line.
• The secondary consists of a large
number of turns of fine wire and
provides for the measuring
instruments and relays a current
which is a constant fraction of the
current in the line.
(ii) Voltage transformer.
• It is essentially a step down transformer and
steps down the voltage to a known ratio.
• The primary of this transformer consists of a
large number of turns of fine wire connected
across the line.
• The secondary winding consists of a few turns
and provides for measuring instruments and
relays a voltage which is a known fraction of
the line voltage.
7. Metering and Indicating Instruments
• Ammeters, voltmeters, energy meters etc
installed in a sub-station to maintain watch
over the circuit quantities
8. Miscellaneous equipment
 (i) fuses
 (ii) carrier-current equipment
 (iii) sub-station auxiliary supplies
Bus-Bar Arrangements in Sub-Stations
(i) Single bus-bar system
• it consists of a single bus-bar and all the incoming
and outgoing lines are connected to it.
• Advantages• low initial cost, less maintenance and simple
operation
• Disadvantage• If repair is to be done on the bus-bar or a fault occurs
on the bus, there is a complete interruption of the
supply
Single Bus Bar System
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(ii) Single bus-bar system with sectionalisation
The single bus-bar is divided into sections and load
is equally distributed on all the sections.
Any two sections of the busbar are connected by a
circuit breaker and isolators.
Advantages- if a fault occurs on any section of the
bus, that section can be isolated without affecting
the supply from other sections
repairs and maintenance of any section of the
busbar can be carried out by de-energising that
section only, eliminating the possibility of complete
shut down.
Sectionalize Bus bar
(iii) Duplicate bus-bar system
• consists of two bus-bars, a “main” bus-bar and
a “spare” bus-bar
• Each bus-bar has the capacity to take up the
entire sub-station load
• Ordinarily, the incoming and outgoing lines
remain connected to the main bus-bar.
However, in case of repair of main bus-bar or
fault occuring on it, the continuity of supply to
the circuit can be maintained by transferring it
to the spare bus-bar.
Duplicate Bus Bar
Terminal and Through Sub-Stations
• Depending upon the manner of incoming
lines, the sub-stations are classified as :
(i) Terminal sub-station
(ii) Through sub-station
(i) Terminal sub-station
• one in which the line supplying to the substation
terminates or ends
• It may be located at the end of the main line or it
may be situated at a point away from main line
route.
(ii) Through sub-station
• one in which the incoming line passes
‘through’ at the same voltage
Key Diagram of 66/11 kV Sub-Station
Key Diagram of 11 kV/400 V Indoor
Sub-Station
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